Firearm Cartridge, Percussion Nipple and Forming Tool Therefor

ABSTRACT

The firearm cartridge has an upper chamber adapted to house gunpowder and a bullet, a lower pocket adapted to house a primer, and a flash hole extending there between. The lower pocket has a ledge disposed generally perpendicularly to the flash hole. The flash hole is formed with a perimeter wall that has a convex curvature adjacent to the ledge of the primer pocket. Likewise, a percussion nipple adapted for use in a black powder rifle has an elongated bore formed with relatively larger and smaller diameter portions connected by a shoulder, and the shoulder has a convex curvature adjacent to the larger diameter portion of the bore. The tool for forming the foregoing shapes has a tip provided with a concave curved surface corresponding to the convex curvature of the flash hole perimeter wall on the cartridge or to the convex curvature of the shoulder in the elongated bore of the percussion nipple.

BACKGROUND

The present invention relates to firearm cartridges and percussion nipples, and more specifically to the flash hole design of either a cartridge used in a modern firearm, or the nipple used in a percussion black powder firearm, and to a specialized drill used to produce these shapes.

An ammunition cartridge typically consists of a brass or steel case having a main chamber filled with a propellant such as smokeless powder and a bullet seated in the front of the chamber. A recessed pocket is provided in a rear face of the cartridge to house a primer. One or two flash holes is/are punched in the wall separating the primer pocket from the chamber containing the propellant. When the primer is detonated, hot gas shoots through the flash hole(s) and into the chamber to ignite the propellant. There are two styles of cartridges. A “Boxer” cartridge has one central flash hole approximately 0.080″ to 0.090″ in diameter and a primer pocket approximately 0.175″ in diameter. A “Berdan” cartridge has two diametrically opposing flash holes approximately 0.050″ in diameter.

The flash hole is typically punched in the primer pocket from outside the cartridge towards the interior. This manufacturing operation occasionally results in a burr on the inside edge of the flash hole. Competitive shooters remove the burr with a special drill called “flash hole uniformer” to counter bore the flash hole from the inside of the cartridge.

As illustrated in FIG. 1, a conventional monolithic ammunition cartridge or round, generally designated 10, basically comprises a cup-shaped primer 12, a case 14, and a bullet 16. The primer contains a shock sensitive explosive 18. The case 14 is formed with an upper chamber 20 that contains a propellant such as smokeless gunpowder 22, a lower pocket 24 adapted to house the primer 12, and a relatively small diameter flash hole 26. The lower pocket 24 is typically defined by a cylindrical side wall 28 and an upper ledge 30 extending inwardly and generally perpendicularly to the side wall 28. The flash hole 26 is typically defined by a cylindrical perimeter wall 32 extending at a right angle from the upper ledge 30 of the primer pocket 24 to the upper chamber 20. When the primer 12 is struck by a firing pin (not shown), the primer explosive 18 is converted to ignition gas 34. The flash hole 26 enables the ignition gas 34 to flow from the primer pocket 24 to the powder chamber 20.

As illustrated in FIG. 2, the ignition gas 34 may experience flow resistance or back flow in the form of eddies or vortices 36 as it passes from the primer pocket 24 into the flash hole 26. It is believed that the sudden restriction imposed by the upper ledge 30 chokes the flow of the gas, thereby diminishing some of its force. It is further believed that if the flow of ignition gas through the flash hole were improved, the combustion of the powder 22 in the upper chamber of the cartridge and the subsequent travel of the bullet 16 would also be improved.

SUMMARY OF THE INVENTION

The present invention constitutes an improvement in a firearm cartridge and a percussion nipple of a black powder rifle, and a tool for providing these improvements. The cartridge may have a conventional upper chamber adapted to house gunpowder and a bullet, a lower pocket adapted to house a primer, and a flash hole extending there between. The lower pocket has a ledge disposed generally perpendicularly to the flash hole. The improvement comprises providing a flash hole that is formed with a perimeter wall having a convex curvature adjacent to the ledge of the primer pocket. In the percussion nipple, an elongated bore is formed with relatively larger and smaller diameter portions connected by a shoulder, and the improvement comprises providing the shoulder with a convex curvature adjacent to the larger diameter portion of the bore. The tool has a tip formed with a concave curved surface corresponding to the convex curvature of the flash hole perimeter wall on the cartridge or to the convex curvature of the shoulder in the elongated bore of the percussion nipple.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of a conventional firearm cartridge;

FIG. 2 is an enlarged fragmentary sectional view of the cartridge shown in FIG. 1 with a chamfer between the primer pocket and the flash hole perimeter wall;

FIG. 3 is a view similar to FIG. 2 and particularly illustrates a curve between the primer pocket and the flash hole perimeter wall;

FIG. 4 is a fragmentary sectional view illustrating an arcuate flash hole perimeter wall;

FIG. 5 is a view similar to FIG. 4 and particularly illustrates a parabolic perimeter wall;

FIG. 6 is a view similar to FIGS. 4 and 5 and particularly illustrates an elliptical perimeter wall;

FIG. 7 is an elevational view of a conventional percussion or flash nipple for a black powder rifle;

FIG. 8 is a sectional view of the flash nipple illustrated in FIG. 7 and particularly illustrates a conventional flash tube therein;

FIG. 9 is an enlarged fragmentary view of the flash tube illustrated in FIG. 8; and

FIG. 10 is a view similar to FIG. 9 of a flash tube having a new shoulder or transition wall configuration.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

In incompressible flow (or gas flow at subsonic speeds of less than about Ma 0.7, where gas can effectively be modeled as incompressible), a constriction in pipe diameter will yield an increase in velocity, since mass flow rate along the pipe is theoretically constant. However, where there is compressible, or supersonic flow, as obtained from the detonation of a high explosive primer, a sudden constriction in a conduit will cause a choking condition or resistance that limits the flow of ignition gas through the constriction. It is believed that the amount of such resistance depends upon the shape of the constriction, particularly the inlet of the constriction. One that is perpendicular to the axis of the conduit will cause a loss that is greater than a chamfered inlet which, in turn, will cause flow loss that is greater than a rounded inlet constriction. As these losses are examined in the context of supersonic flow of ignition gas in a firearm cartridge, the flash hole may be seen as a supersonic convergent nozzle, where the negative effect of the change from a large diameter to a small diameter on fluid flow may be mitigated by the shape of the constriction.

In order to allow the greatest and fastest flow of gas from the primer chamber to the powder chamber, it is therefore desired that the flash hole create the least amount of back pressure possible. To this end, it is proposed that the generally perpendicular intersection of the flash hole perimeter wall 32 and the primer pocket upper ledge 30 illustrated in FIG. 1 be eliminated in favor of, at the very least, a chamfer 40 as illustrated in FIG. 2. It is believed that the chamfer 40 would act as a funnel for the primer gasses trying to pass through and ignite the powder. However, for supersonic flow, depending on the exact velocity, less obstruction would be provided by a curved entryway that becomes tangent to the through-hole, but does not make a full 90 degree ‘quarter round’ profile, perhaps making only a 45 or a 60 degree swing, creating something of a trumpet shape. A step better would be a flash hole entryway that is rounded with a circular profile on the primer side. As illustrated in FIG. 3, a flash hole perimeter wall 132 may be formed with a convex or rounded shape to streamline the ignition gas flow 34 and reduce flow resistance from the primer pocket 24 into the powder chamber 20.

Further, and depending on the gas parameters, other profiles which are not round might be utilized to advantage. Parabolic, elliptical, power series, Von Karmen, Haack, or many other shapes might be optimum. And again, any of these profiles might swing through a full 90 degrees, but might also only swing through a reduced angle. As illustrated in FIG. 4, one of the possible profiles of the flash hole perimeter wall 232 may be a segment of a circle 238. The position and size of the arc 238 may be varied so that it is tangent to the flash hole at the inlet 240, and/or tangent to the flash hole at the outlet 242. FIG. 5 illustrates a flash hole perimeter wall 332 that is a segment of a parabola 338. The parabolic segment 338 maybe be tangent to the flash hole inlet 340 and/or outlet 342. FIG. 6 Illustrates a flash hole perimeter wall 432 that is a segment of an ellipse 438. The elliptical segment 432 profile may be tangent to the flash hole inlet 440 and/or outlet 442.

The same or similar principle may be applied to flash nipples for black powder rifles and pistols. FIGS. 7-9 illustrate a conventional percussion or flash nipple 544 for black powder rifles. An elongated flash hole or tube 526 is formed therein with an intermediate, conically-shaped transition shoulder or wall 546 between larger 526A and smaller 526B diameter portions of the tube. A cup shaped primer (not shown) is placed over the larger diameter portion of the flash tube. The smaller diameter passage is of sufficient diameter to allow the ignition gasses to ignite the powder, but remains small enough to limit the back-flow of gas from inside the barrel. In this case, the transition shoulder 526 typically has a slight chamfer to it, based on the fact that most flash nipples are manufactured by drilling, and drills have a taper point to them. In this application, a trumpet shaped transition can ease the flow of primer gasses. A tangent ogive, parabola, ellipse, Von Karmen, Haack, or power series profile might all be used to advantage.

FIG. 10 illustrates how the transition area 646 could be improved by using an inwardly curved transition wall 646. Not shown, but illustrated in concept in previous illustrations cited above are the various curves which could be used to create curved transition shoulders or walls, including but not limited to circular arcs, elliptical arcs, parabolic arcs, power series arcs, Von Karmen profile or Haack profile arcs, or other mathematically defined curves.

The third element to this patent is the shaped drill which is so shaped that when it is used to drill a hole, the tip profile is one of the aforementioned shapes of a parabola, ellipse, Von Karmen profile, Haack profile, or power series profile.

A further embodiment of the flash hole design can be seen in the application of this invention to the percussion nipples of a black powder rifle. In these rifles, a tubular hollow nipple protrudes from the breech, wherein the powder is packed behind the bullet. Instead of a primer which is inserted into the primer pocket of a brass case, as described above, a percussion cap is placed over the tip of the nipple, and is struck directly with the hammer. The nipple thus acts as the anvil, such that the compressive shock generated between the hammer and the anvil will detonate the primer compound. The primer nipple is designed such that there is initially a large bore, allowing the hot gases from the primer detonation to flow relatively unobstructed, allowing fast propagation of the gas. However, at the end of the nipple, there is a reduction of diameter of the bore. This is necessary, so that once the powder in the breech is ignited, the high pressure in the barrel does not cause significant back-flow out the nipple, leading to a loss of pressure behind the bullet, or potentially blowing back the hammer, causing it to be damaged. It is desired that this smaller bore be of generally short length, in order to minimize the flow restriction of the primer gasses in the direction passing into the breech where the powder is stored. A low flow restriction into the breech is desired in order to get maximum ignition reliability. Typical for nipple construction is that the large bore, on the order of 0.075″ to 0.100″, is drilled to partial depth, and then a smaller bore, on the order of 0.025″ to 0.035″ is drilled through. Using this manufacturing technique, the transition of the large bore into the small bore will have a steep chamfer, or conical transition, with an included angle typical of a drill bit used for machining. That is to say, an included angle of 118 degrees or 135 degrees.

Applying this invention to a black powder flash nipple, it can be seen that replacing the conical transition from the large bore to the small bore, having a 135 or 118 degree included angle, with a smooth transition, would improve flow. This smooth transition could be in the form of a DeLaval supersonic nozzle design, it could also use a trumpet shape which has a sudden onset of the converging section, but then comes tangent to the small diameter bore. As described above, the transition could use a surface based on the revolution of a circular arc, or a parabolic, or elliptical arc, or an arc from some other mathematically derived curve.

It is contemplated that the foregoing convex perimeter wall of the flash hole and the convex shoulder of the flash nipple may be formed with a drill or end mill. The tip of the tool would have a concave curved surface which corresponds to any of the profiles described above, such that the drill can be used to drill or mill, or otherwise cut the various flash hole profiles into a cartridge or into a flash nipple. 

1. In a firearm cartridge having an upper chamber adapted to house gunpowder and a bullet, a lower pocket adapted to house a primer, a flash hole extending between the upper chamber and the lower pocket, said lower pocket having a ledge disposed generally perpendicularly to the flash hole, said flash hole being defined by a perimeter wall, that improvement which comprises: said flash hole perimeter wall having a convex curvature adjacent to the ledge of the primer pocket.
 2. The firearm cartridge according to claim 1, wherein the convex curvature of said perimeter wall is a mathematically derived shape.
 3. The firearm cartridge according to claim 2, wherein the mathematically derived shape of said perimeter wall is one of a set comprising arcuate, parabolic, elliptical, Von Karmen, Haack, and power series.
 4. The firearm cartridge according to claim 1, wherein the convex curvature of said perimeter wall extends from the primer pocket ledge to the upper chamber of the cartridge.
 5. In a percussion nipple adapted for use in a black powder rifle, said percussion nipple having an elongated bore formed with relatively larger and smaller diameter portions connected by a shoulder, that improvement which comprises: said shoulder having a convex curvature adjacent to the larger diameter portion of the bore.
 6. The percussion nipple firearm cartridge according to claim 5, wherein the convex curvature of said shoulder is a mathematically derived shape.
 7. The percussion nipple according to claim 6, wherein the mathematically derived shape of said shoulder is one of a set comprising arcuate, parabolic, elliptical, Von Karmen, Haack, and power series.
 8. The percussion nipple according to claim 5, wherein the convex curvature of said shoulder extends from the larger diameter portion to the smaller diameter portion of said elongated bore.
 9. A tool for forming one of a set comprising: a firearm cartridge having an upper chamber adapted to house gunpowder and a bullet, a lower pocket adapted to house a primer, a flash hole extending between the upper chamber and the lower pocket, said lower pocket having a ledge disposed generally perpendicularly to the flash hole, said flash hole being defined by a perimeter wall, and said flash hole perimeter wall having a convex curvature adjacent to the ledge of the primer pocket; and a percussion nipple adapted for use in a black powder rifle, said percussion nipple having an elongated bore formed with relatively larger and smaller diameter portions connected by a shoulder said tool having a tip formed with a concave curved surface corresponding to the convex curvature of the flash hole perimeter wall of the firearm cartridge or to the convex curvature of the shoulder in the elongated bore. 